1. Field of the Invention
The present invention relates to passive optical modulation type display apparatuses in which the amount of light emitted from the light source can be changed.
2. Description of the Related Art
A conventional passive optical modulation type display apparatus employs a light modulation device, to modulate in response to image data inputted from outside, light emitted from the light source and thereby display images. By adjusting light modulation device optical transmittance or its reflectance, the display apparatus of the passive optical modulation type displays gray scale of the image data. The fact that the light modulation device in this display apparatus has inferior light modulation linearity and stray light enters the device, will worsen a contrast ratio of maximum to minimum brightness, so that the gray scale that the image data intrinsically retains are not allowed to be displayed.
For this reason, the contrast ratio is improved by adjusting the intensity of light—the amount of light—that enters the light modulation device. For instance, there is a method for controlling the amount of light from the light source, in response to features of inputted image data. The peak brightness that represents the features of the image data is detected, so that the contrast ratio is enhanced by intensifying the light source when the detected peak brightness is high, or by lowering it when low (refer to, e.g., Japanese Unexamined Patent Publication 62-19835, page 4, FIG. 1, and Japanese Unexamined Patent Publication 6-160811, page 2, FIG. 1). Alternatively, the average brightness that represents the features of the image data is detected, so that the contrast ratio is enhanced by intensifying the light source when the average brightness is high, or by lowering it when low (refer to, e.g., Japanese Unexamined Patent Publication 5-66501, page 3, FIG. 1, and U.S. Pat. No. 5,717,422, page 15, FIG. 8). This method is employed in direct-view liquid crystal displays using, as backlight, cold-cathode tube fluorescent lamps or light-emitting diodes.
The conventional passive optical modulation type display apparatus adopts as the light source a high intensity discharge (HID) lamp such as a metal halide lamp or an ultrahigh pressure mercury lamp. The HID lamp uses tungsten as an electrode material. When the lamp is lit, a halogen cycle is utilized in order to reduce tungsten wear due to arc-discharging heat. Proper operation of the halogen cycle requires maintaining bulb inner wall temperature within a proper temperature range. However, when the amount of light emitted from the source is adjusted in accordance with the image data features, adjusting the amount of light based on electric power fed to the lamp will greatly change the bulb inner wall temperature, in response to the electric power variations, as well. As a result, a problem has been that when the wall temperature falls outside the proper temperature range, the halogen cycle will not work properly, thus resulting in a lamp lifetime being reduced.
Furthermore, a spectral distribution characteristic of radiant light from the HID lamp is greatly dependent on the pressure of in-bulb substances. The pressure of the in-bulb substances is in turn dependent on the in-bulb temperature. Consequently, another problem has been that when the wall temperature lies outside the temperature range, the spectral distribution of the radiant light will vary, thus resulting in color characteristics, such as lamp color temperature and chromaticity, being changed. Furthermore, discharging impedance in the HID lamp is largely dependent on the in-bulb pressure and temperature. Therefore, another problem has been that when the wall temperature goes out of the temperature range, the discharging impedance will vary, which causes a load on the electrode to change, resulting in the lamp lifetime being reduced. Furthermore, when the lamp works properly so that the wall temperature remains within the temperature range, the amount of light emitted from the light source cannot be greatly changed. Therefore, another problem has been that the contrast ratio cannot be greatly improved.
The present invention is directed to overcome the above-discussed problems. It is an object of the present invention to provide a passive optical modulation type display apparatus that ensures a prolonged lamp lifetime and the proper lamp operation even when higher contrast is realized in response to features of inputted image data, by adjusting the amount of light from the light source. Another object of the present invention is to provide a passive optical modulation type display apparatus enhancing the contrast ratio by adjusting the amount of light that enters the light modulation device.